Prolonged fasting-induced metabolic signatures in human skeletal muscle of lean and obese men

Abstract

Insulin resistance is a well-known physiological adaptation to prolonged fasting in healthy skeletal muscle. Obesity is associated with insulin resistance and metabolic inflexibility in skeletal muscle, and a pronounced increase in the risk of metabolic complications. Under the hypothesis that the metabolic traits of insulin resistance associated with prolonged fasting are different from insulin resistance associated with obesity, we examined nine obese and nine lean participants during 12 and 72h of fasting, respectively. Insulin resistance in obese participants was associated with impaired insulin signaling, and reduced levels of glucose-6-phosphate and TCA-cycle intermediates. 72h of fasting in lean participants reduced insulin-stimulated glucose uptake to levels similar to obese participants fasted for 12h. This was associated with increased lipid oxidation, but not accumulation of diacylglycerol or acylcarnitines and impairment of insulin signaling. Prolonged fasting was associated with pronounced increases in β-hydroxybutyrate and β- hydroxybutyrylcarnitine levels in skeletal muscle suggesting augmented ketone body metabolism. Fasting induced insulin resistance may be a consequence of substrate competition. The underlying mechanism behind insulin resistance in obesity is thus not comparable to the physiological adaptations in skeletal muscle induced by prolonged fasting in lean participants.

Fig 1. Glycogen content and regulators/markers of metabolism determined after 12 and 72 h of fasting.

Data are presented as mean±SD. A: Glycogen levels were significantly reduced by 72 h of fasting (fasting effect P<0.001) and tended to be lower in the obese group than the lean (BMI effect P = 0.062). B: There were three 2-way interactions between the effects of BMI, fasting and insulin on rate of whole body glycogen synthesis measured as non-oxidative glucose disposal (NOGD). NOGD significantly increased by insulin (P<0.001). C: 72 h of fasting increased phosphorylation of GS p-Ser⁶⁴¹ reflecting decreased activity of the enzyme (fasting effect P = 0.012), whereas insulin increased GS activity (insulin effect P<0.001). D: Representative Western blots. *P<0.05 compared to non-insulin-stimulated conditions, ⁺P<0.05 compared to lean, ^#P<0.05 compared to 12 h fast.

Fig 2. Protein levels and phosphorylations of Akt, AS160, hexokinase, AMPK and ACC.

Phosphorylation of Akt and AS160 were assessed by Western blots in muscle biopsies taken before (black bars) and during (grey bars) insulin stimulation after 12 and 72 h of fasting in lean and obese participants. Protein levels of Hexokinase II, as well as phosphorylations of AMPK and ACC were assessed in non-insulin-stimulated muscle biopsies from lean (white bars) and obese participants (crossed bars) after 12 and 72 h of fasting. Data are presented as mean±SD. A and B: Insulin stimulation significantly increased phosphorylation on both pAKt Ser⁴⁷³ and Thr³⁰⁸ regardless of BMI and duration of the fast. We found significant BMI x insulin and fasting x insulin interactions on both phosphorylation sites. Post hoc tests revealed that insulin-stimulated Akt phospylation was lower in obese than lean after 12 h of fasting (P<0.01) but increased during 72 h of fasting. In lean, Akt phosphorylation levels were not affected by fasting. C: We found an interaction between the effect of insulin and fasting (fasting x insulin P = 0.01) on pAS160 Thr⁶⁴²/AS160, and post hoc test revealed a lower insulin-stimulated phosphorylation level in obese after 12 h of fasting compared to lean (P = 0.047). Similar to Akt, the insulin-stimulated levels in obese increased during 72 h of fasting to levels comparable to lean. D: Protein levels of Hexokinase II (relative to total amount of protein) were lower in obese than lean (P = 0.016). E: pAMPK Thr¹⁷²/AMPK decreased slightly during 72 h of fasting with no difference between groups (fasting effect P = 0.048) F: pACC/ACC was not significantly affected by BMI or fasting. Representative blots in panel D, E and F have been cut to remove insulin-stimulated biopsies. *P<0.05 compared to non-insulin-stimulated conditions, ⁺ P<0.05 compared to lean, ^# P<0.05 compared to 12 h fast.